A variety of electric squibs have been developed as squibs for gas generators for inflating air bags equipped in cars.
Such a squib usually has metal pins for electrically connecting to the external, and a heating element at the other ends of the metal pins for igniting an explosive.
Hitherto used ignition devices have used bridging wires for igniting explosive, and nichrome wires have been used as bridging wires. If wire diameters are too fine, the bridging wires could not be attached. However, when using bridging wires of diameters enabling the wires to be attached, the wires of such diameters generally have a large heat capacity so that the time period becomes longer from turning on electricity to the moment at which the bridging wires reach the ignition temperature of the explosive, with the result that, for example, the quick responsibility required for side inflators for cars is not sufficient.
In order to obtain another ignition device, moreover, a method has been known for forming a thick film resistor directly on a printed circuit board using the technique for producing the printed circuit board.
For example, Patent Document 1 disclosed a squib using a printed circuit board on which a thick film resistor is directly formed, and a varistor is mounted on another part on the printed circuit board for the purpose of electrostatic protection. Moreover, Patent Document 2 likewise disclosed a squib obtained by production steps of mounting a resistive heating element on a printed circuit board, connecting a capacitor and a varistor to the printed circuit board by soldering, and further connecting the product thus obtained to electrode pins.
Although these techniques have improved the quick responsibility from the level of those using bridging wires, the responsibility has remained insufficient.    Patent Document 1: Official Gazette of Japanese Patent Application Laid Open No. 2003-205,823    Patent Document 2: Official Gazette of Japanese Patent Application Laid Open No. 2000-108,838
On the other hand, the semiconductor bridge (SCB) is a generic name of bridges produced using the semiconductor technique such as sputtering, vapor deposition, or the like. Ignition devices using semiconductor bridges are of a very fine structure having fine line widths in comparison with those using bridging wires or printed circuit boards. Further, the ignition devices using semiconductor bridges utilize thin film bridges whose film thicknesses are of the order of a few microns so that it is possible to make their heat capacity smaller so as to have quick responsibility. While the bridging wires require time period of 800 to 1,000 microseconds for heating ignition powder to its ignition temperature with electric current of 1.2 A, the semiconductor bridges generally enable ignition powder to be ignited only requiring time period of the order of 100 to 200 microseconds. With the SCB, since semiconductor production equipment such as for sputtering and vapor deposition may be used, it is possible to make the board remarkably smaller and further possible to make the heat capacity of heating part smaller and to accurately control the heat capacity, thereby stably producing squibs higher in ignition responsibility.
Following methods for installing such an SCB in the squib have been known. The SCB is directly mounted on a header, and end electrodes are connected to electrode pins by wire bonding. Or, as disclosed in Patent Document 3, similarly to the printed circuit board, once the SCB is mounted on a printed circuit board, and after the printed circuit board has been mounted on a header, electrode pins and predetermined electrodes of the printed circuit board are connected by soldering.    Patent Document 3: Official Gazette of Japanese Patent Application Laid Open No. 2002-13,900.
When using such a semiconductor bridge having a smaller heat capacity, however, there is a risk of the squib erroneously igniting due to noise such as external static electricity.
Consequently, known in the art is arranging a capacitor in parallel with the semiconductor bridge for absorbing discharged static electricity in order to prevent the erroneous ignition due to the static electricity.
A plan view of FIG. 1 (a) and a sectional view of FIG. 1 (b) illustrate a typical example of the squib of the type that a heating resistive element is directly formed on a printed circuit board as a base for igniting, or an SCB is mounted on a board and an electrostatic protection device is provided at another location of the board.
As shown in these drawings, according to the hitherto used method, generally, a printed circuit board 1 is formed with through-holes, and electrode pins are brought into direct contact with the printed circuit board 1 and connected thereto by soldering or the like. Nickel chrome alloy is frequently used as a thick film resistor 2. In order to connect an electrostatic protection device 3, for example, varistor, capacitor, or diode as electrostatic protection means in parallel with the thick film resistor 2, electrodes are provided on the printed circuit board 1 at the location at which the electrostatic protection device 3 is to be mounted, and the electrostatic protection device 3 is fixed thereto by soldering in the same manner as general circuits. In this method, although the electrode pins 4 can be readily connected to the thick film resistor 2 and the electrostatic protection device 3, the miniaturization of squibs has been limited, since the printed circuit board 1 itself is connected to the electrode pins 4.